Watching a sand mould being made

Last week in
Metal Casting, Part 1 we
saw how patterns and sand moulds are made for metal casting. Those patterns (and
moulds) varied from being very simple to much more complex, the latter including
cores, risers and runners.

This week we’re going to watch a sand mould being
prepared from a pattern, the work being done to produce an alloy aftermarket V8
intake manifold. To be able to follow the process, we suggest that you first
read Part 1 of this series – otherwise, it’ll
be really easy to get lost.

The moulding was being carried out at the Matilda
Foundry in Maryborough, Queensland. The Matilda Foundry specialises in small
production runs in non-ferrous materials, including aluminium and bronze. We
watched owner Neil Anderson at work. Neil has more than 30 years experience in
casting.

The sand work for the alloy inlet manifold was
made in four parts –

the upper mould

the lower mould

the core for the runners

the core for the undercentre

The final result is an amazing three-dimensional
jigsaw puzzle!

The first step was to retrieve the pattern from
storage. Patterns are made by dedicated tradespeople called patternmakers. The
pattern is critical to the success of the casting, not only in that it needs to
create the correctly-shaped final product but also that it need to be able to be
withdrawn from the sand mould, be durable, and so on. Some patterns that Neil
has seen have been completely unworkable. The pattern was first placed on the
joint board; this plane will become the parting line of the mould.

A removable wooden moulding box surrounds the
upper part of the pattern. The part of the pattern that can be seen replicates
the top surface of the intake manifold.

The surface of the pattern was lightly dusted with
talcum powder. This allows the pattern to be more easily withdrawn from the sand
mould.

Next Neil filled a container with the special
moulding sand. The machine adds the chemical binders to the sand as it is being
dispensed. Scales are used so that the correct mass of sand for each mould is
dispensed.

The sand was then poured into the moulding box. At
this stage it flows just like normal, fine-grained sand – however it is
hardening all the time. Neil suggested that on a hot day, he has to work fast to
get the sand properly packed in the mould. Even on this cool day, he still
worked fast!

Using gloved fingers, he carefully packed the sand
around the pattern, pushing it into openings and tamping it into place. Sand
filled the moulding box almost to the top surface. The assembly was then placed
to one side for the sand to go hard.

The next step was to produce the cores. This is
the core box for the undercentre of the mould. It will produce the shaped sand
core that prevents metal flowing into the space under the runners.

Sand is packed into the small cavities of the
corebox...

...and then the corebox is closed and sand is
packed into the closed volume.

The sand core for the intake manifold runners is
then produced.

By this stage, the mould that we saw being formed
earlier has hardened, and the mould can be flipped over and the joint board
removed. It then looks...

....like this. Note the circles marked on the
pattern – we’ll come back to their function in a moment.

This side of the pattern is dusted with talcum
powder and then...

...this side of the pattern is boxed.

The same sand-packing process is again followed,
starting with the addition of a measured quantity of sand, followed by....

..packing and then the placement of brass
‘chills’, located on the previously shown circles. Chills are pieces of metal placed within the mould that promote faster
cooling of the casting at these points. This quickens the rate at which thick
portions of the casting cool, keeping the overall solidification of the casting
occurring at a more even rate. (The chills are later retrieved from the broken
sand mould.)

After the sand has set, the runner core box can be
parted, revealing the runner sand core.

The runner core pattern comes apart (the arrowed
parts removed) to allow the core to be withdrawn complete and undamaged.

The complete sand core for the runners. It’s now
much clearer how this shaped sand core represents the ‘induction air passages’
portion of the intake manifold.

The other core that we saw being made was the
undercentre core. This has also now hardened and can be withdrawn from its core
box.

The sand core for the runners is then cleaned up,
a small trowel being used to remove the joint fin (the line left by the parting
face of the core box) from the core.

The bottom of the mould can now also be removed,
the sand having hardened.

The bottom surface of the mould, complete with
brass chills, can now be seen in its finished form. It needs only to be removing
from its moulding box.

The pattern is then removed from the upper portion
of the mould. In this and the above pic you can now see the upper and lower
internal surfaces of the mould.

The underside of the top section of the mould
shows four openings. These are for two runners and two risers – that is, where
the molten aluminium will be poured in and where it will rise up far enough to
nearly overflow.

The mould and cores can now be assembled. The
underside core is firstly placed on top of the lower section of the mould.

I must admit to getting lost at this point so Neil
placed the finished casting (a Funnelweb 2V 351 Ford V8 inlet manifold by Parker
Racing) next to the incomplete mould. Looking at this pic, you can see how the
underside core (arrowed) represents the lower spaces of the manifold not filled
with alloy.

Next the runner core can be placed into position.
Note how this core fits into recesses in the lower mould, so allowing proper
registration.

The mould is then closed by the placement of the
upper half of the mould.

Finally, the mould for the runners and risers is
rested on top. It is through two of these holes that next week we’ll see the
liquid metal being poured.

Conclusion

As you can see, the complexity of moulds and cores
can move very quickly from something as simple as the ‘cotton reel’ shaped
casting we saw the pattern and mould for last week, to a sand mould consisting
of four separate, interlocking parts. The complexity of a cylinder head mould
can only be imagined!

Next week, in the last part in this series, we’ll
watch the intake manifold being poured and then the sand mould being broken open to
reveal the results.